The prior art is documented with examples of receiver extensions, such as incorporated into a firearm in communication with a rear extending shoulder or butt stock. The receiver tube is also termed a buffer tube and functions to allow the user to attach a stock to the rifle as well as to encapsulate the buffer and buffer spring, letting the bolt carrier ride back and forth to eject the spent brass and load the next round.
As is also known, the buffer and buffer spring are housed within the buffer tube and are crucial elements for the AR's cyclic action. The buffer is the contact point for the bolt carrier to move rearward within the buffer tube and the spring resides in the most aft position of the assembly. When the buffer spring recoils, it pushes the buffer and bolt carrier forward, thus loading the next round of ammunition and closing the bolt for the next shot.
Walters, U.S. Pat. No. 8,397,414, teaches an M4/AR15 adjustable and multi-position receiver extension with an add-in part (see buttstock pre-adjustment block 10). Chvala, U.S. Pat. No. 8,955,245, teaches another version of an adjustable stock for a firearm and which, of note, includes a movable member provided with locking structure mounted on the stock for selective engagement with lock receiving structure on the buffer tube for locking the stock in various axial adjustment positions along the buffer tube.
U.S. Pat. No. 9,664,476, to Robinson (RMDI, LLC), teaches an adjustable firearm stock with first and second selector bodies configured within a position selector device integrated into interface between the receiver tube and displaceable stock (see FIGS. 3-4), this differing in numerous respects from the proposed design.
US 2018/00807636 to Olsen teaches an adjustable buttstock which may be attached to a buffer tube with a plurality of position slots. The adjustable buttstock may include a body slidably attached to the buffer tube, a butt attached to the body, and a lever assembly rotatably attached to the body.
Also noted is the Magpul AR15/M4 Enhanced Receiver Extension which includes a plurality of apertures configured in spaced apart fashion upon an extending underside thereof. The receiver tube is secured to a rear end of the lower receiver and so that a projecting end of the receiver tube is seated within an interior of a shoulder stock. A trigger style lever is integrated into an interior window of the shoulder stock and, when depressed, permits the stock to be repositioned along the length of the receiver tube over several positions by reseating a pin integrated into the lever within one of the spaced apart underside apertures configured within the receiver tube.